1. Field of the Invention
This invention relates to a surface acoustic wave filter device where an elastic surface element and a housing are assembled particularly by face down bonding techniques.
2. Description of the Related Art
As is generally known, surface acoustic wave filter devices are compact and light and have a high performance and high reliability in terms of functions, so that they have been widely used in various fields.
Two-way transmission filters for mobile communication are required to present high attenuation characteristics to the frequency blocking regions in their frequency characteristics. One two-way transmission filter is so constructed that a plurality of inter-digital transducers (hereinafter, referred to as IDTs) are arranged in the direction in which elastic surface waves propagate and mode-coupling two-port surface acoustic wave filters using the multiple resonance mode are connected in a multistage manner (a surface acoustic wave filter device).
To improve the frequency characteristic of a surface acoustic wave filter device, a method of connecting a one-port resonator in series with mode-coupling two-port surface acoustic wave filters can be considered.
In this case, the one-port resonator is used as a notch filter. When the attenuation pole of the one-port resonator is set near the high-pass region of the frequency pass band of the mode-coupling two-port surface acoustic wave filters, the entire frequency characteristic presents a sharp cut-off characteristic in the high-frequency region.
Here, the inventors of the present invention directed their attention to the fact that the addition of a one-port resonator causes a new problem. The inventors concentrated their attention on the fact that the problem particularly arises in a surface acoustic wave filter device where component parts are assembled by face down bonding techniques. In a surface acoustic wave filter device using face down bonding techniques, the notch characteristic of the one-port resonator cannot be obtained in the desired frequency region as designed, with the result that the entire frequency pass band characteristic of a surface acoustic wave filter device superior in quality cannot be obtained.
It is accordingly an object of the present invention to provide a surface acoustic wave filter device which enables the notch characteristic of a one-port resonator to be obtained reliably near the high-pass region of the frequency pass band of a mode-coupling two-port surface acoustic wave filter, even when component parts are assembled by face down bonding techniques.
Another object of the present invention is to provide a surface acoustic wave filter device which enables unnecessary stray capacitance components in a one-port resonator to be reduced sufficiently and realizes a frequency pass band characteristic excellent in flatness.
To accomplish the foregoing objects, an embodiment of the present invention is configured as follows. In a surface acoustic wave filter device where the electrodes of an elastic surface wave element are connected via bumps to the electrodes of a housing by face down bonding techniques, the elastic surface wave element has a first two-port surface acoustic wave filter, a one-port elastic surface wave resonator filter to which the output of the first surface acoustic wave filter is supplied, a second two-port surface acoustic wave filter to which the output of the one-port elastic surface wave resonator filter is supplied, elastic-surface-wave-element side signal input and output electrodes, and an elastic-surface-wave-element side ground electrode each formed on one face of a piezoelectric substrate. In addition, the housing has signal input and output electrodes and a ground terminal electrode corresponding to the elastic-surface-wave-element side signal input and output electrodes and the elastic-surface-wave-element side ground electrode, the ground electrode having an opening section facing the one-port elastic surface wave resonator filter.